Frequency modulation receiver



DETEcTa/P c. TRAVIS Fil'ed July 2, 1942 FREQUENCY MODULATION RECEIVER //PL /F/E/F Feb. 13, 1945.

I Y I.- /PEQl/l/PED @HND Patented Feb. 13, v1945 y 2,359,621; mEQUENoY'MonUnA'rioN Receiver;

Charles-` Travis; deceased, lateof` Wyndmoor; Pat, liyr Yvonne' Travis; adminlstratrix;-V Wyndmoxm.

- Pa:,.= assiznor.:

Gorporatiom. y of.- Delaware.l

to` Philco Radio f andt'lelevlsion. Bhiladelphimt Pa., a corporation Application July 1942;.Sel'laliNo:.449 ;506'

' comme. (crete-2o);

- Thisjiuvo'rltion-rclatcsf. to: Irequenoyimodiilatiori.

receivers',.andA ,more particularly to" an. improved intermediate frequency transformer, and.v to' thel comlinatonjl `of an improved intermediate-`v fre-V quency .transformen and'. an amplitude limiter.'I

In frequency modulation receivers. equipped with. amplitude limiters'irregu1r ities in ampie mation-and incfrequency' response. character-.

` l'gand a1suitable2soundtoutput device H. Thel present` inventioni relatesA to'lfaynovel design andr construction .ofthe"intermediatefrequency transformers 2 4 and' 6; and: to thecombination yof lsaidtransforiners'iwiththeamplitude limiter stage 1. The?. family of` curves of'4 lig.y 2` illustrates the l well-.knowny variation of. frequency yresponse with isti'cs.areiremoved'bythe.limiter,.and"hence, are

of.: littlezimportance in. the. detectedresultl- BY thefpresen-t .invenlim advantage is taken'. of'ttiis factL in .the :adjustment of.'one. or more of the in"- y termedia-te frequency coupling .transformers to' provideincreased Aselectivity' and the maximum- Dossiblegain; r l t It\=i5...aprincipa1. object of4 this invention. to provide. intermediate. frequency. amplifier;

y and-limiter.- oombnationfad'aptedfor usefin fre'- quencyfmodulation receivers, having" maximum gain anddmprovedf.selectivity, characteristics. ,l

couplingwior" the" case of undamped,r doubletunedjbandpass. circuits of the type .shown in Fig-f. 1i. at@ 4'J andi Withy loose coupling,.i. et,

` below.-Y critical' coupling,.-the frequency response It is a further object of 'the invention tollrovide' anintermediate frequency. transiimner.` unit havngihishfgam. but low manufacturing cost, andhadapted. for. use inI frequency'` modulation radio receivers.. y

The invention' itself as well" as other` 4collects thereof, will.' be..und'arstqod` from the. following description and.- theY accompanying. drawing.' 'in which:

. .'Fig. l is a schematic, andpartiallykdia'graini matmiuustration .ofa frequency modulation re- `mit@ of.' the., superheterodyn type' ellbOdYlg the. present invention;

Fig..r 2.. variousyfre'quency .response characteristics ob- .tainable.by varying the couplingv of' an' interm'e; diatefequency transformer; q A s f Fig. I.3' .illustrates thepreferredy frequency` retsponse-` characteristic'. of a transformer' con.- structed'iii. accordancewith theinvention; andJA ,'san explanatory diagram" ill'l'islratine characteristicffof curveais` typical. Thiscurve isv` characterized'. by having, a single, relatively sharp?. peaki at thefresonance frequency. .The

pass-band 'ofi' suchan '-.under-coupled `.transformer is veryI narrow; andrat 20 kilocycles -oiresonance thefgainr of. this particular transformer'is only aismallnfractionof. its.4 gain:Y at` resonance; l vThe curve b'f illustrates? the transformers 'frequency response characteristic for'A the condition of critical: coupling, i; ve., where` ythe coecient of coupling: is equal to.1 the.Z reciprocal4 of thev square root of the product of the Qs of the coupledcircuits.. Curve c. shows the.` frequency response characteristic for `a slightly.. over-coupled condition, and is characterizedlby two response peaks, onelon. either. side of..Y resonance, with .a valley or hollow between. the. peaks;at resonance. By a further.. increase.. oi thecoefllcient of coupling.,

the frequency. response. characteristic of ',curve .d

is A secured, .andwith -a..still. further increase .in l

coupling, the response. characteristic illustrated by-the..curve.-e maybe obtained, etc. The curves denoted Qd. and .e represent ,theA condition of strong over-coupling.` 1

Accordingto oneV of thefeatures ofthe invention the couplingbetween the .windings of .the in termediate frequency transformer is tightened to such antextent that the transformer.4 gain at the Fig; 4.illustratesthe response characteristic" of lReference. i`s-now made t'o-Fig. 1 in ,which there yis. illustrated a frequency .modulation radio: re"- .ceiveroft the superheterodyne. type' embodying the presentinvention; receivermay comprise a suitable'fantenna rl .aL converter. stage c'cmifzrising thegnrst detector 2 andthe local oscillator; 3, yan

intermediate frequency: coupling; transformera an intermedatefrequency. amplier 5f, a secondk intermediatefrequenoy coupling transformer 6, enfsmnlitudje-limiter- Stage-1;@ third intermediste yifre'qutncyooup1ingr-- transformer, .8, a second va.. typicalM intermediate. frequency transformer Yconstituetes.in .accordance .with .the prorart.'

edges of the. useful bandis substantially. equal to thetransformergainat resonance. Assume, by may of examplethat. an intermediate frequency coupling transformer is to be designed for use in a frequencymcdulation receiver, `and that a total intermediate frequency band-width `of 200 kilocyclesfis required; By reference to the respouse curvesoi'liiggi` 2; takenfor different values detector stage;9,. an audio. frequency .amplifier 455 coupling,l it isl evident that an equality of transformer 'gain' at resonance and at 100" kilorcycles off resonance is'provided by the curve e, aifdlthis'- represents the* preferred characteristic in' acicordance"v the present invention. It shouldbe understood; however, that the invention' `isgnot' lmit'edtothis precise equality,. it bein'g suflcient that this -relationslii'pA be only approximately met. For a 150 kilocycle band-width (one extending 75 kilocycles on either side of resonance) a curve lying intermediate curves e and d would preferably be selected.

In Figs. 3 and 4, there is afforded a comparison of the relative gain and selectivity characteristics of the improved transformer of the-present invention and of a typical transformer of the prior art, respectively. In the gain characteristic il#`- lustrated in Fig. 3, the coupling between the transformer windings has been adjusted to the point where the gain at resonance is equal to the gain at the edges of the required frequency band. The band is denoted by the cross-hatched area below the curve, and the maximum useful gain is designated x. 'I'he response characteristic of Fig. 4 is usually obtained through the agency of a transformer having damped and slightly overcoupled windings, similar to the one illustrated at 8 in Fig. 1. This transformer, with its damping resistors removed, may have a characteristic similar to that designated c in Fig. 2. By the application of the damping resistors, however, the gain is reduced at all frequencies, but largely. in the vicinity of the two peaks and at resonance,

resulting in the substantially flat-top bandfpass characteristic of Fig. 4. Here again the useful stage gain is that provided at the outer edges of the required frequency band, illustrated by the cross-hatched area underneath the curve. In practice, the response of the` transformer is further attened by the provision of an amplitude limiter followingthe transformer, thus giving a substantially uniform gain over the required band. In Fig. 4 this gain is denoted y. From a consideration of the response characteristics of Figs. 3 and 4 in the light of the general principles illustrated in Fig. 2. it is evident that gain :1: will always be greater than gain y, and, in fact, that the gain :1: is the greatest value of gain that may be reached as the coupling between the coils is increased.

It is also evident from an examination of the curves of Figs. 2, 3 and 4 that the selectivity of a transformer constructed in accordance with the present invention will be greater than that of a transformerconstructed in accordance with the'prior art. This is most clearly evident from Fig. 2, where it is apparent that the curve e has a steeper slope in the vicinity of 100 kilocycles oil resonance than do any of the other curves.

It is apparent, of course, that the gain of a transformer of the character described varies greatly over the required band, with the result that a frequency-modulated signal, upon passage through the transformer, will emerge as a fresarily followed. `The relative effects of changes in coil Qs and in the coefficient of couplings between the coils is too well known to require detailed comment. For the general laws relating to the various constants of coupled circuits of this character, reference may be had to Terman,

Radio Engineering, second edition, 1937, pages I In considering the application of the present invention to a radio receiver of the superheterodyne type, reference may be had to Fig. 1 which has already been described in general terms.

f Transformers 4 and 6, constructed in accordance with the principles of the present invention, may be employed as the coupling means between the first detector stage 2 and the intermediate frequency amplier 5, and between the said amplifier stage and the limiter stage 1. If additional intermediate frequency amplifier stages are provided, similar transformers may advantageously be employed in the couplings between the several stages. If transformer coupling is employed between the limiter stage and the second detector stage, and no additional amplitude limiting means are associated with the said transformer,

' it is preferred to employ, at this point, a transformer of the conventional type, having a band-- Y pass characteristic of the character illustrated in 40 present invention without departing from the ouency-modulated signal with pronounced amplitude modulation. However, by providing an amplitude limiter to follow the over-coupled transformer. and adjusted to limit approximately alongthe line M-N of Fig. 3, this undesired amplitude modulation is eliminated along with other amplitude modulation components vwhich it is the limiters function to remove.

In the construction of the transformer of the present invention, it is desirable, in order to secure a maximum gain, to employ windings having a relatively high Q, and to avoid external forms of damping. The elimination of external damping resistors 'or other damping means, results, of course, in a simpler transformer of decreased manufacturing cost. The Q of the primary coils may conveniently be made equal to the Q of the secondary coils, but this procedure is not necesspirit thereof.

VAlthough the invention has been described with particular reference to a specific embodiment, it will be understood that the invention is capable of general application and isadapted to other forms of physical expression and is, therefore, not to be limited by this specific disclosure, but only by the scope of the appended claims.

What is claimed is: K.

1. In a frequency modulation receiver of the superheterodyne type, an intermediate frequency transformer comprising tightly-coupled primary and secondary windingsmeans for tuning said windings to produce an overall transformer responsive characteristic having a pronounced resonant peak on each side of the receivers intermediate frequency and substantially equidistant therefrom, the coupling between said tuned windings being such that the transformer gain at the point of minimum gain between peaks is substantially equal to the transformer gain on the outer slopes of said peaks at the edges of the receivers intermediate frequency band, an amplitude limiter following said transformer and operative to limit the signal amplitude to a level substantially corresponding to said point of minimum gain, a detector circuit following said amplitude limiter, and a ,coupling device having a substantially flat-top band-pass characteristic connected between said limiterand said detector.

2.111 a frequency modulation'receiver of the superheterodyne type, an intermediate frequency -amplifier whose frequency response characteristicv has a pronounced resonant peak on each side of the receivers intermediate frequency and sub'- stantially equidistant` therefrom, the amplier gain at the point of minimum gain between peaks being substantially equal to the amplier gain ony the outer-slopes of said peaks at the edges of the receivers intermediate frequency band, an amplitude limiter` following said amplifier and operative to limit the signal amplitude to a level substantially corresponding to said point of minimum gain, a detector circuit following said amplitude limiter, and a coupling device having a substantially flat-top band-pass characteristic connected between said limiter and said detector.y

frequency transformers, the overall frequency response characteristic of said amplier having a pronounced resonant peak on each side of the receiversiintermediate frequency and substantially equidistant therefrom, the amplier gain at the point of minimum gain between peaks being substantially equal to the amplier gain on the outer slopes of said peaks at the edges of the receivers intermediate frequency band, and an amplitude 3. In a frequency modulation'receiver of the Y superheterodyne type, an intermediate frequency amplifier including a plurality of intermediate 15 Deceased. 

